So I’m just going to dump my experience here hoping it helps somebody.
This is Step 1 – making your Pi’s be able to ping each other and communicate via UDP/TCP, ad-hoc, peer to peer, without any central router. This doesn’t include making them route packets with 2 or more hops between origin and destination.
Long story short I’ve tried like a hundred different things and as usual a simple thing made it work.
So here it started as a weekend / hobby project trying to learn more about mesh networking and mesh communications. I’ve purchased 2 x Raspberry Pi 2 Model B (below) along with all standard stuff including usb Wi-Fi dongles and set sail with a goal of simply making them ping each other and eventually transfer a file in a mesh / ad-hoc network mode, basically so that none of them is an access point and both aren’t connected to any 3rd access point.